Why that Person Left You
There are many reasons; that's number one. There's not one singular cause that led to that person walking out of the relationship, however, there is one major understanding that will ameliorate your suffering.
And all that matters is your response to that person leaving, not the fact that he or she left.
So, here is one truth (and it's a big one) to let in; depending on your comfort level with metaphysics and the spiritual realm, your ego may reject this notion immediately, but let it settle in; see if there isn't the tiniest part of you - under the currents of your conscious mind - that knows that this is true: The relationship was no longer serving you, and he or she was supposed to leave.
Here's the thing that the world has wrong (and it's quite an enormous concept as it relates to our notion of self-worth and happiness in the area of love), which is that not every relationship is meant to last a lifetime. That ideology is from the world. That ideology was created at a time when people's life spans were much shorter. That ideology was created when people's consciousness and psyches weren't changing at such a rapid pace. Today, we are all evolving quickly. We meet someone who is an atheist who doesn't want kids, and two years later, he or she wakes up early to do buddhist chants and volunteer at an after school program. The person who was perfect for us at 25 is the wrong partner for us at 37, and it has nothing at all to do with us being unloveable or not good enough; rather, it is is the natural order of things; it is your soul's call for expansion.
Yes, there are people who expand together and change in tandem, allowing for a constant evolution of themselves and of the relationship, which means their partnership can last a lifetime, but this is not every relationship. The longevity of our relationships are dependent upon the evolution of our consciousness and our intrinsic craving for expansion.
The world would have us believe that we should stay with the first love we encounter, namely for mundane reasons such as security and procreation, but oftentimes, that diminishes our capacity for joy, as we don't allow a new partner - one who mirrors back our current level of consciousness - to enter our life.
You are perfect just as you are. That person had to leave because you were no longer growing with him or her. Your soul wanted more. How does that idea resonate with your spirit? Does it make you feel lighter, like a weight has been lifted? If so, then let is serve as a balm when you are feeling depressed, or questioning your self-worth.
There is divine order. The universe is not a chaotic, random setup. Whoever has left your life has left for a reason. It is for your own good. Trust that. Allow yourself to believe that all things are working out for your highest good.